1. Field of the Invention
The present invention relates to an image forming apparatus that primarily transfers a toner image formed on an image bearing member to an intermediate transfer medium and secondarily transfers the toner image on the intermediate transfer medium to a sheet. Particularly, the present invention relates to velocity control of the image bearing member and the intermediate transfer medium.
2. Description of the Related Art
Conventionally, a color image forming apparatus of an electrophotographic system, such as a color copier, forms toner images on surfaces of image bearing members (photosensitive drums) for respective colors, and primarily transfers these toner images to an intermediate transfer medium. Then, the superimposed toner images that are primarily transferred to the intermediate transfer medium are secondarily transferred to a sheet to form a color image.
In such an image forming apparatus, in order to prevent deviation (color misregistration) among the color toner images, it is necessary to keep the peripheral velocities of the photosensitive drums constant and to keep the peripheral velocity of the intermediate transfer medium constant. Japanese Laid-Open patent publication (Kokai) No. S62-178982 (JP S62-178982A) discloses a technique that controls angular velocity of a drum shaft with a feedback control on a DC motor as a driving source using a rotational velocity sensor arranged on the drum shaft of a photosensitive drum in order to rotate the photosensitive drum at a constant velocity.
In the configuration where toner images formed on the photosensitive drums are transferred to the intermediate transfer medium, it is preferable that the relative velocity (peripheral velocity difference) between the photosensitive drums and the intermediate transfer medium becomes zero in consideration of an effect on image expansion and contraction of the image. However, even if the angular velocity of the drum shaft is controlled with the technique disclosed in JP S62-178982A, the velocity fluctuation due to mechanical shape errors, such as backlash of gears, error in roundness and eccentricity of the photosensitive drum, cannot be avoided. Then, if such velocity fluctuation causes the peripheral velocity difference, friction torque will occur between the photosensitive drum and the intermediate transfer medium, and this friction torque will cause further velocity fluctuation. When the velocity of either of the photosensitive drum and the intermediate transfer medium varies under the condition where the peripheral velocity difference is set so as to become zero especially, the large and small relation of the peripheral velocities (namely, plus/minus sign of the peripheral velocity difference) is not fixed, which enlarges the range of fluctuation of the friction torque caused therebetween.
In order to control bad influence of the velocity fluctuation, Japanese Laid Open Patent Publication (Kokai) No. H4-324881 (JP H4-324881A) discloses a technique that always generates one way friction torque by setting the peripheral velocity of the photosensitive drum being higher than that of the intermediate transfer medium. Thereby, even if the velocity fluctuation occurs, the large and small relation of the peripheral velocities is fixed, which enables to control color misregistration by making the range of fluctuation of the friction torque small.
Moreover, Japanese Laid-Open Patent Publication (Kokai) No. H6-317992 (JP H6-317992A) discloses a technique that sets the peripheral velocity of the intermediate transfer medium being higher than that of a photosensitive drum in order to improve transcriptional efficiency.
However, when the peripheral velocity difference is set between the photosensitive drum and the intermediate transfer medium as mentioned above, a slower one of the photosensitive drum and the intermediate transfer medium in the peripheral velocity is pulled by the other of which the peripheral velocity is faster, which decreases the drive torque of the motor of the slower one. When a toner image arrives at a primary transfer unit under such a condition, since the friction torque between the photosensitive drum and the intermediate transfer medium increases, the decreased driving torque of the motor sharply increases, which greatly changes the peripheral velocity of the slower one of the photosensitive drum and the intermediate transfer medium for a while. That is, when the toner image arrives at the primary transfer unit in an image formation starting period and when the toner image goes away from the primary transfer unit in an image formation ending period, the friction torque varies sharply, which greatly fluctuates the rotational velocity of a DC motor that is a driving source. The velocity fluctuation may cause a remarkable color misregistration image.